A convention cooling apparatus (Japanese Patent Application Laid-Open (kokai) No. 2000-161803) is constructed as shown in FIG. 9. A superconductive magnet 101 cooled by means of a first refrigerant 103a such as liquid helium is accommodated within a vessel 102. The vessel 102 is fixed to a vacuum tank 107 via a large number of heat insulating support members 104, a shield plate 105, and a large number of heat insulating support members 106. Vapor of the first refrigerant 103a such as liquid helium is condensed to liquid by means of a first cooling unit 110.
A second cooling unit 250 includes a refrigerant circulation circuit 250A and a pulse tube refrigerator 250B. The refrigerant circulation circuit 250A consists of a liquid reservoir 251 fixed to the vacuum chamber 107 through a large number of heat insulating support members 254 and storing a second refrigerant liquid 253a1 such as liquid nitrogen; and a conduit 252 receiving the second refrigerant liquid 253a1 within the liquid reservoir 251, being in thermal contact with the shield plate 105, and returning to a second refrigerant gas phase portion 253b of the liquid reservoir 251.
The pulse tube refrigerator 250B consists of a compressor 250B1 and a second low-temperature generating section 250B2. High-pressure piping 264 of the second low-temperature generating section 250B2 communicates with high-pressure ports of rotary changeover valves 253a and 253b connected to a drive section 274. Low-pressure piping 263 of the second low-temperature generating section 250B2 communicates with low-pressure ports of the rotary changeover valves 253a and 253b. 
Communication ports of the rotary changeover valves 253a and 253b communicate with a cold reservoir 255 and an atmospheric-temperature-side throttle 260, respectively. A condenser 256a is provided on a low-temperature side of the cold reservoir 255. The condenser 256a communicates with a condenser 256b provided on a low-temperature side of a pulse tube 258 via a conduit 257. An atmospheric-temperature side of the pulse tube 258 communicates with the throttle 260 via a radiator 259. The high-pressure piping 264 and the low-pressure piping 263 of the second low-temperature generating section 250B are connected to the compressor 250B1 through high-pressure piping 262 and low-pressure piping 261, respectively.
In the above-described cooling apparatus, the pulse tube and the cold reservoir are of substantially the same length. However, when a low temperature to be generated is about 100 K or lower, efficiency lowers unless the length of the pulse tube is at least about three times the length of the cold reservoir. When the length of the pulse tube is set to at least about three times the length of the cold reservoir in order to improve efficiency, a portion of the pulse tube, from the cold end to a point near the midpoint, is immersed in the second refrigerant. As a result, heat is conducted from the pulse tube to refrigerant, accompanied by occurrence of a problem of a lowered rate of condensation of refrigerant vapor.
Moreover, when the cold end of the pulse tube is positioned in the second refrigerant gas phase portion, the pulse tube projects from the vacuum tank by a greater amount as compared with the cold reservoir, accompanied by occurrence of a problem of an increased occupation space of the cooling apparatus.